4,927 research outputs found
Spitzer/MIPS Imaging of NGC 650: Probing the History of Mass Loss on the Asymptotic Giant Branch
We present the far-infrared (IR) maps of a bipolar planetary nebula (PN), NGC
650, at 24, 70, and 160 micron taken with the Multiband Imaging Photometer for
Spitzer (MIPS) on-board the Spitzer Space Telescope. While the two-peak
emission structure seen in all MIPS bands suggests the presence of a near
edge-on dusty torus, the distinct emission structure between the 24 micron map
and the 70/160 micron maps indicates the presence of two distinct emission
components in the central torus. Based on the spatial correlation of these two
far-IR emission components with respect to various optical line emission, we
conclude that the 24 micron emission is largely due to the [O IV] line at 25.9
micron arising from highly ionized regions behind the ionization front, whereas
the 70 and 160 micron emission is due to dust continuum arising from
low-temperature dust in the remnant asymptotic giant branch (AGB) wind shell.
The far-IR nebula structure also suggests that the enhancement of mass loss at
the end of the AGB phase has occurred isotropically, but has ensued only in the
equatorial directions while ceasing in the polar directions. The present data
also show evidence for the prolate spheroidal distribution of matter in this
bipolar PN. The AGB mass loss history reconstructed in this PN is thus
consistent with what has been previously proposed based on the past optical and
mid-IR imaging surveys of the post-AGB shells.Comment: 9 pages in the emulated ApJ format with 6 figures, to appear in Ap
The Circumstellar Extinction of Planetary Nebulae
We analyze the dependence of circumstellar extinction on core mass for the
brightest planetary nebulae (PNe) in the Magellanic Clouds and M31. We show
that in all three galaxies, a statistically significant correlation exists
between the two quantities, such that high core mass objects have greater
extinction. We model this behavior, and show that the relation is a simple
consequence of the greater mass loss and faster evolution times of high mass
stars. The relation is important because it provides a natural explanation for
the invariance of the [O III] 5007 planetary nebula luminosity function (PNLF)
with population age: bright Population I PNe are extinguished below the cutoff
of the PNLF. It also explains the counter-intuitive observation that
intrinsically luminous Population I PNe often appear fainter than PNe from
older, low-mass progenitors.Comment: 12 pages, 2 figures, accepted for ApJ, April 10, 199
Optical Identification of Close White Dwarf Binaries in the LISA Era
The Laser Interferometer Space Antenna (LISA) is expected to detect close
white dwarf binaries (CWDBs) through their gravitational radiation. Around 3000
binaries will be spectrally resolved at frequencies > 3 mHz, and their
positions on the sky will be determined to an accuracy ranging from a few tens
of arcminutes to a degree or more. Due to the small binary separation, the
optical light curves of >~ 30% of these CWDBs are expected to show eclipses,
giving a unique signature for identification in follow-up studies of the LISA
error boxes. While the precise optical location improves binary parameter
determination with LISA data, the optical light curve captures additional
physics of the binary, including the individual sizes of the stars in terms of
the orbital separation. To optically identify a substantial fraction of CWDBs
and thus localize them very accurately, a rapid monitoring campaign is
required, capable of imaging a square degree or more in a reasonable time, at
intervals of 10--100 seconds, to magnitudes between 20 and 25. While the
detectable fraction can be up to many tens of percent of the total resolved
LISA CWDBs, the exact fraction is uncertain due to unknowns related to the
white dwarf spatial distribution, and potentially interesting physics, such as
induced tidal heating of the WDs due to their small orbital separation.Comment: 4 pages, 2 figure
The Environments around Long-Duration Gamma-Ray Burst Progenitors
Gamma-ray burst (GRB) afterglow observations have allowed us to significantly
constrain the engines producing these energetic explosions. Te redshift and
position information provided by these afterglows have already allowed us to
limit the progenitors of GRBs to only a few models. The afterglows may also
provide another observation that can place further constraints on the GRB
progenitor: measurements telling us about the environments surrounding GRBs.
Current analyses of GRB afterglows suggest that roughly half of long-duration
gamma-ray bursts occur in surroundings with density profiles that are uniform.
We study the constraints placed by this observation on both the classic
``collapsar'' massive star progenitor and its relative, the ``helium-merger''
progenitor. We study several aspects of wind mass-loss and find that our
modifications to the standard Wolf-Rayet mass-loss paradigm are not sufficient
to produce constant density profiles. Although this does not rule out the
standard ``collapsar'' progenitor, it does suggest a deficiency with this
model. We then focus on the He-merger models and find that such progenitors can
fit this particular constraint well. We show how detailed observations can not
only determine the correct progenitor for GRBs, but also allow us to study
binary evolution physics.Comment: 44 pages including 11 figure
Modeling lithium rich carbon stars in the Large Magellanic Cloud: an independent distance indicator ?
We present the first quantitative results explaining the presence in the
Large Magellanic Cloud of some asymptotic giant branch stars that share the
properties of lithium rich carbon stars. A self-consistent description of
time-dependent mixing, overshooting, and nuclear burning was required. We
identify a narrow range of masses and luminosities for this peculiar stars.
Comparison of these models with the luminosities of the few Li-rich C stars in
the Large Magellanic Cloud provides an independent distance indicator for the
LMCComment: 7 pages, 2 figure
Effects of a burst of formation of first-generation stars on the evolution of galaxies
First-generation (Population III) stars in the universe play an important
role inearly enrichment of heavy elements in galaxies and intergalactic medium
and thus affect the history of galaxies. The physical and chemical properties
of primordial gas clouds are significantly different from those of present-day
gas clouds observed in the nearby universe because the primordial gas clouds do
not contain any heavy elements which are important coolants in the gas.
Previous theoretical considerations have suggested that typical masses of the
first-generation stars are between several and
although it has been argued that the formation of very massive stars (e.g., ) is also likely. If stars with several are most popular
ones at the epoch of galaxy formation, most stars will evolve to hot (e.g.,
K), luminous () stars with gaseous and dusty
envelope prior to going to die as white dwarf stars. Although the duration of
this phase is short (e.g., yr), such evolved stars could contribute
both to the ionization of gas in galaxies and to the production of a lot of
dust grains if the formation of intermediate-mass stars is highly enhanced. We
compare gaseous emission-line properties of such nebulae with some interesting
high-redshift galaxies such asIRAS F10214+4724 and powerful radio galaxies.Comment: 25 pages, 7 figures, ApJ, in pres
Observational Tests and Predictive Stellar Evolution II: Non-standard Models
We examine contributions of second order physical processes to results of
stellar evolution calculations amenable to direct observational testing. In the
first paper in the series (Young et al. 2001) we established baseline results
using only physics which are common to modern stellar evolution codes. In the
current paper we establish how much of the discrepancy between observations and
baseline models is due to particular elements of new physics. We then consider
the impact of the observational uncertainties on the maximum predictive
accuracy achievable by a stellar evolution code. The sun is an optimal case
because of the precise and abundant observations and the relative simplicity of
the underlying stellar physics. The Standard Model is capable of matching the
structure of the sun as determined by helioseismology and gross surface
observables to better than a percent. Given an initial mass and surface
composition within the observational errors, and no additional constraints for
which the models can be optimized, it is not possible to predict the sun's
current state to better than ~7%. Convectively induced mixing in radiative
regions, seen in multidimensional hydrodynamic simulations, dramatically
improves the predictions for radii, luminosity, and apsidal motions of
eclipsing binaries while simultaneously maintaining consistency with observed
light element depletion and turnoff ages in young clusters (Young et al. 2003).
Systematic errors in core size for models of massive binaries disappear with
more complete mixing physics, and acceptable fits are achieved for all of the
binaries without calibration of free parameters. The lack of accurate abundance
determinations for binaries is now the main obstacle to improving stellar
models using this type of test.Comment: 33 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Integral-Field Spectroscopy of the Post Red Supergiant IRC +10420: evidence for an axi-symmetric wind
We present NAOMI/OASIS adaptive-optics assisted integral-field spectroscopy
of the transitional massive hypergiant IRC +10420, an extreme mass-losing star
apparently in the process of evolving from a Red Supergiant toward the
Wolf-Rayet phase. To investigate the present-day mass-loss geometry of the
star, we study the appearance of the line-emission from the inner wind as
viewed when reflected off the surrounding nebula. We find that, contrary to
previous work, there is strong evidence for wind axi-symmetry, based on the
equivalent-width and velocity variations of H and Fe {\sc ii}
6516. We attribute this behaviour to the appearance of the complex
line-profiles when viewed from different angles. We also speculate that the Ti
{\sc ii} emission originates in the outer nebula in a region analogous to the
Strontium Filament of Carinae, based on the morphology of the
line-emission. Finally, we suggest that the present-day axisymmetric wind of
IRC +10420, combined with its continued blueward evolution, is evidence that
the star is evolving toward the B[e] supergiant phase.Comment: 22 pages, 9 figures, accepted for publication in ApJ. B&W-optimized
version can be downloaded from http://www.cis.rit.edu/~bxdpci/pubs.htm
Chemical Abundances of Planetary Nebulae in the Sagittarius Dwarf Elliptical Galaxy
Spectrophotometry and imaging of the two planetary nebulae He2-436 and
Wray16-423, recently discovered to be in the Sagittarius dwarf elliptical
galaxy, are presented. Wray16-423 is a high excitation planetary nebula (PN)
with a hot central star. In contrast He2-436 is a high density PN with a cooler
central star and evidence of local dust, the extinction exceeding that for
Wray16-423 by E(B-V)=0.28. The extinction to Wray16-423, (E(B-V)=0.14), is
consistent with the extinction to the Sagittarius (Sgr) Dwarf. Both PN show
Wolf-Rayet features in their spectra, although the lines are weak in
Wray16-423. Images in [O III] and H-alpha+[N II], although affected by poor
seeing, yield a diameter of 1.2'' for Wray16-423 after deconvolution; He~2-436
was unresolved. He2-436 has a luminosity about twice that of Wray16-423 and its
size and high density suggest a younger PN. In order to reconcile the differing
luminosity and nebular properties of the two PN with similar age progenitor
stars, it is suggested that they are on He burning tracks
The abundance pattern is very similar in both nebulae and shows an oxygen
depletion of -0.4 dex with respect to the mean O abundance of Galactic PN and
[O/H] = -0.6. The Sgr PN progenitor stars are representative of the higher
metallicity tail of the Sgr population. The pattern of abundance depletion is
similar to that in the only other PN in a dwarf galaxy companion of the Milky
Way, that in Fornax, for which new spectra are presented. However the
abundances are larger than for Galactic halo PN suggesting a later formation
age. The O abundance of the Sgr galaxy deduced from its PN, shows similarities
with that of dwarf ellipticals around M31, suggesting that this galaxy was a
dwarf elliptical before its interaction with the Milky Way.Comment: 24 pages, Latex (aas2pp4.sty) including 5 postscript figures. To
appear in Ap
A Spitzer IRS Spectral Atlas of Luminous 8 micron Sources in the Large Magellanic Cloud
We present an atlas of Spitzer Space Telescope Infrared Spectrograph (IRS)
spectra of highly luminous, compact mid-infrared sources in the Large
Magellanic Cloud. Sources were selected on the basis of infrared colors and 8
micron (MSX) fluxes indicative of highly evolved, intermediate- to high-mass
stars with current or recent mass loss at large rates. We determine the
chemistry of the circumstellar envelope from the mid-IR continuum and spectral
features and classify the spectral types of the stars. In the sample of 60
sources, we find 21 Red Supergiants (RSGs), 16 C-rich Asymptotic Giant Branch
(AGB) stars, 11 HII regions, 4 likely O-rich AGB stars, 4 Galactic O-rich AGB
stars, 2 OH/IR stars, and 2 B[e] supergiants with peculiar IR spectra. We find
that the overwhelming majority of the sample AGB stars (with typical IR
luminosities ~1.0E4 L_sun) have C-rich envelopes, while the O-rich objects are
predominantly luminous RSGs with L_IR ~ 1.0E5 L_sun. We determine mean
bolometric corrections to the stellar K-band flux densities and find that for
carbon stars, the bolometric corrections depend on the infrared color, whereas
for RSGs, the bolometric correction is independent of IR color. Our results
reveal that objects previously classified as PNe on the basis of IR colors are
in fact compact HII regions with very red IRS spectra that include strong
atomic recombination lines and PAH emission features. We demonstrate that the
IRS spectral classes in our sample separate clearly in infrared color-color
diagrams that use combinations of 2MASS data and synthetic IRAC/MIPS fluxes
derived from the IRS spectra. On this basis, we suggest diagnostics to identify
and classify, with high confidence levels, IR-luminous evolved stars and HII
regions in nearby galaxies using Spitzer and near-infrared photometry.Comment: 46 pages, 9 figures; accepted for publication in AJ; abstract
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